Friday, 26 May 2023

Dynamic method dispatch Or runtime polymorphism in Java

Dynamic method dispatch, also known as runtime polymorphism, is a feature in object-oriented programming that allows a program to determine which implementation of a method to invoke at runtime, based on the actual type of the object being referenced. It enables method calls to be resolved dynamically at runtime rather than being determined at compile-time.

Dynamic method dispatch is closely related to inheritance and overriding. When a subclass overrides a method declared in its superclass, the subclass provides its own implementation of that method. This means that objects of the subclass can be treated as objects of the superclass, and the overridden method can be invoked through a reference of the superclass type. However, the specific implementation of the method that is executed is determined by the actual type of the object being referenced.

class Animal {
    public void makeSound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    public void makeSound() {
        System.out.println("Dog barks");
    }
}

class Cat extends Animal {
    public void makeSound() {
        System.out.println("Cat meows");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal animal1 = new Animal();
        Animal animal2 = new Dog();
        Animal animal3 = new Cat();

        animal1.makeSound();  // Output: Animal makes a sound
        animal2.makeSound();  // Output: Dog barks
        animal3.makeSound();  // Output: Cat meows
    }
}
 

In this example, we have a superclass Animal and two subclasses Dog and Cat. Each class has its own implementation of the makeSound() method. In the main() method, we create objects of the different classes and assign them to variables of the type Animal.

When the makeSound() method is called on animal1, which references an Animal object, the implementation from the Animal class is invoked. However, when the same method is called on animal2 and animal3, which reference Dog and Cat objects, respectively, the overridden implementations from the respective subclasses are invoked. This is dynamic method dispatch in action, where the specific method implementation is determined by the actual type of the object being referenced, rather than the declared type of the variable.

Dynamic method dispatch allows for more flexible and polymorphic behavior in object-oriented programming, enabling objects of different types to be treated uniformly through their common superclass or interface, while still invoking their specific behavior at runtime.


In Java, upcasting and downcasting are related to the concepts of inheritance and polymorphism, and they involve the manipulation of object references.

  1. Upcasting: Upcasting refers to the process of assigning a reference of a subclass to a reference variable of its superclass. It is an implicit conversion that happens automatically. Upcasting is safe because a subclass object inherently has all the properties and behaviors of its superclass.
  1. Downcasting: Downcasting is the opposite of upcasting. It involves casting a reference of a superclass to a reference variable of its subclass. Since a superclass reference can refer to a subclass object, downcasting is required to access the specific members (methods or fields) of the subclass.

However, downcasting is not automatically done and requires an explicit cast. It is important to note that downcasting can only be performed when the actual object being referenced is of the target subclass type. Otherwise, a ClassCastException will be thrown at runtime.

Inheritance and Constructors in Java: is constructor inherited in Java?

Understanding how the constructors work within the inheritance idea is crucial. The constructors are never passed down to any child classes during inheritance.

In Java, a parent class's default constructor is automatically called by the constructor of its child class. That implies that the parent class constructor is called first, followed by the child class constructor, when we create an object of the child class.

class A {
    A()
    {
        System.out.println("Super class constructor");
    }
    
}

class B extends A
{
    B()
    {
       System.out.println("\n Sub class constructor");
    }
}

class Demo
{
    public static void main(String args[])
    {
        B obj1 =new B();
        
    }
}

 Output:

 Super class constructor

 Sub class constructor

However, only the default constructor will be automatically invoked by the child class constructor if the parent class contains both a default and a parameterized constructor.

The parametrized constructor of the super class is called using super keyword. The point to note is base class constructor call must be the first line in the derived class constructor.

class A {
    int x;
    A(int x)
    {
        System.out.println("Super class constructor");
        System.out.println(x);
    }
    
}

class B extends A
{
    B(int x)
    {  super(x);
       System.out.println("\n Sub class constructor");
       System.out.println(x);
    }
}

class Demo
{
    public static void main(String args[])
    {
        B obj1 =new B(10);
        
    }
}

Output:

Super class constructor
10

 Sub class constructor
10

Wednesday, 24 May 2023

Opensource Operating Systems

 The study of operating systems has been made easier by the availability of a vast number of open-source releases.

Open-source operating systems are those available in source-code format rather than as compiled binary code.

Learning operating systems by examining the source code has other benefits as well.

With the source code in hand, a student can modify the operating system and then compile and run the code to try out those changes, which is an excellent learning tool.

There are many benefits to open-source operating systems, including a community of interested (and usually unpaid) programmers who contribute to the code by helping to debug it, analyze it, provide support, and suggest changes.

Arguably, open-source code is more secure than closed-source code because many more eyes are viewing the code.

Companies that earn revenue from selling their programs often hesitate to open-source their code, but Red Hat and a myriad of other companies are doing just that and showing that commercial companies benefit, rather than suffer, when they open-source their code.

You can use and adapt a variety of open-source operating systems depending on your requirements. Listed below are a few well-liked open-source operating systems:

1. Linux: Linux is a well-known, widely-used open-source operating system. There are numerous different distributions (distros) for it, including CentOS, Ubuntu, Fedora, Debian, and many more. Linux has a large development and contributor community and is very adaptable.

2. FreeBSD: Based on the Berkeley Software Distribution (BSD), FreeBSD is an open-source operating system. It is renowned for its dependability, effectiveness, and security attributes. Servers, embedded systems, and networking equipment all frequently use FreeBSD.

3. OpenBSD: Another open-source operating system with an emphasis on security, accuracy, and code simplicity is OpenBSD. It is renowned for its comprehensive code auditing and proactive attitude to security. Security-sensitive systems and firewall appliances frequently use OpenBSD.

4. NetBSD: A highly portable open-source operating system, NetBSD can be used on a variety of hardware, including desktop computers, servers, and embedded devices. It places a strong emphasis on portability, performance, and clean code.

5. ReactOS is an open-source operating system with the goal of being Windows compatible. It offers an alternative to the Windows operating system and is made to run Windows programmes and drivers. Although ReactOS is still in development, Windows compatibility is promising.

These are but a handful of the many open-source operating systems that are available. Each operating system has distinct advantages, areas of attention, and communities that support it. You can look through them and pick the one that best satisfies your requirements and tastes.

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